The role of humic substances in the sunlight initiated interfacial electron transfer reactions involving organic and inorganic pollutants in natural aquatic systems has been elucidated in the presence of stable semi-conductors as models of minerals with semiconducting properties. The following case studies are reported- i) the sunlight induced reduction of Cr(VI) (chromate anions) on ZnO, in which the sensitizing effect of humic acids was first demonstrated, occurring through either electron or energy transfer to the oxide; ii) the photoinduced reduction of U(VI) on titanium dioxide, investigated by time-resolved fluorescence measurements, in which the rate of the aqueous phase reaction was also enhanced by U(VI) complexation with humic acids, while a photosensitization effect on the interface reaction was evidenced only under high intensity irradiation, filtering effects prevailing at low irradiation; iii) the TiO₂-mediated, sunlight induced degradation of tetrachloroethene. In this case humic acids decreased the rate of the major oxidative route leading to mineralization of the substrate and increased the rate of the reductive route leading to the formation of dichloroacetic acid, which also underwent oxidative photodegradation.